;************************************************************************
;+
;*NAME:
;
;    BITTEST       (General IDL Library 01) 30-MAR-83
;
;*CLASS: 
;
;    Utilities
;
;*CATEGORY:
;
;*PURPOSE:
;
;    TO TEST BIT N IN FIX(X)
;
;*CALLING SEQUENCE:
;
;    BITTEST,X,N,YESNO
;
;*PARAMETERS:  
;
;    X      (REQ) (I) (0 1) (I  F)
;           X is the variable to be tested
;   
;    N      (REQ) (I) (0) (I)
;           The bit of X to be tested
;
;    YESNO  (REQ) (O) (0 1) (I)
;           The result of the test. 1(true) if bit N is set, 0(false)
;           otherwise.
;
;*EXAMPLES:
;
;        BITTEST,!X.STYLE,4,YESNO
;        IF YESNO THEN PRINT,'X-axis suppressed' $
;                 ELSE PRINT,'Draw X-axis'
;
;        To find points in NEWSIPS which are outside calibrated region:
;        BITTEST,ABS(NU),1,YESNO             ; look for nu flag = -2
;        IND = WHERE (YESNO EQ 0)            ; keep points where yesno = 0
;        PLOT,W(IND),F(IND)                  ; plot calibrated points
;        
;*SYSTEM VARIABLES USED:
;
;*INTERACTIVE INPUT: 
;
;*SUBROUTINES CALLED:
;
;    PARCHECK
;
;*FILES USED:
;
;*SIDE EFFECTS: 
;
;*RESTRICTIONS:
;
;*NOTES:
;
;     Note that negative integers are stored in twos complement form.
;     Therefore, the left-most bits are ON rather than OFF as they are for 
;     positive numbers. Input the absolute value of X is negative numbers 
;     to avoid this problem. 
;
;     This procedure can be used to check the values of complex
;     IDL system variables such as ![XYZ].STYLE.
;
;	tested with IDL Version 2.1.0 (sunos sparc)  	19 Jun 91
;	tested with IDL Version 2.1.0 (ultrix mispel)	N/A
;	tested with IDL Version 2.1.0 (vms vax)      	09 Mar 93
;  
;*PROCEDURE:
;
;     Checks whether (FIX(X) OR (NOT 2^N)) = -1 to set the output
;     flag YESNO.
;
;*MODIFICATION HISTORY:
;
;     Mar  6 1983 RJP GSFC initial program
;     Aug 24 1987 RWT GSFC add PARCHECK
;     Mar  7 1988 CAG GSFC all VAX RDAF-style prolog
;     Jul 13 1990 RWT GSFC Sun mods: use examples pertinent to SUN IDL
;                 and add listing of procedure call statement
;     Jun 19 1991 PJL GSFC cleaned up; tested on SUN and VAX; updated prolog
;     Mar  8 1993 RWT GSFC modify to allow X & YESNO to be vectors and 
;                 add documentation about negative numbers.
;-
;*********************************************************************
 pro bittest,x,n,yesno
 if n_params() eq 0 then begin
    print,' BITTEST,X,N,YESNO'
    retall
 endif  ; n_params()
 parcheck,n_params(0),3,'BITTEST'
 yesno = ( fix(x) or (not 2^n) ) eq -1  
; if ( (fix(x) or (not 2^n)) eq -1) then yesno = 1 else yesno=0
 return
 end  ; bittest